1. Field of the Invention
The present invention relates to a novel fluorine-containing copolymer. More particularly, it relates to a copolymer for an elastomer having excellent low temperature resistance and alcohol resistance which comprises specific contents of vinylidene fluoride units and specific fluorovinyl ether units.
2. Description of the Prior Art
It has been considered to utilize alcohol fuel and alcohol-containing fuel because of shortage of petroleum source. Alcohol-containing gasoline as fuel for cars has been practically used in U.S.A. and Brazil. In near future, the change of fuel source will be considered in Japan. In such circumstance, an elastomer having high alcohol resistance as well as high heat resistance, low temperature resistance and oil resistance is needed as an elastomer for cars.
Various known fluorine-containing elastomers have satisfactory heat resistance and oil resistance among these required characteristics. The fluorine-containing elastomer having superior low temperature resistance has not satisfactory alcohol resistance whereas the fluorine-containing elastomer having superior alcohol resistance has not satisfactory low temperature resistance. Any elastomer having entirely satisfactory required characteristics has not been known.
The inventors have studied fluorine-containing elastomers having excellent low temperature resistance and alcohol resistance in the consideration of the aforementioned problems. As a result, it has been found that specific copolymers of vinylidene fluoride and a specific fluorovinyl ether have satisfactory required characteristics.
The present invention has been attained by the finding and is to provide a copolymer for an fluorine-containing elastomer having excellent low temperature resistance and alcohol resistance which comprises vinylidene fluoride units and fluorovinyl ether units having the formula CF.sub.2 .dbd.CFOX (X represents a C.sub.3 -C.sub.9 perfluoroalkyl group which has ether bond having 1-3 oxygen atom) at a molar ratio of vinylidene fluoride units to fluorovinyl ether units of 95:5 to 50:50 and at a total content of the units of at least 80 mole %.
In the present invention, it is important to use the fluorovinyl ether having the formula CF.sub.2 CFOX wherein X represents a perfluoroalkyl group which has 3-9 preferably 4-6 carbon atoms and ether bond having 1-3 preferably 1-2 oxygen atom.
It is not desired to use a fluorovinyl ether having the formula CF.sub.2 CFOX wherein X is a hydrogen-containing group, for example, an alkyl group such as methyl, ethyl and n-butyl group, an alkoxy-alkyl group such as 2-methoxyethyl group, or 2,2,2-trifluoroethyl group, because of inferior heat resistance of the product.
In the case of X which does not include ether bond, if carbon atoms of the monomers are less, the copolymer has not satisfactory alcohol resistance whereas if carbon atoms of the monomers are much enough for improvement of alcohol resistance, the copolymer has inferior low temperature resistance and the copolymer having high molecular weight is not obtained because of inferior polymerization activity.
Suitable perfluorovinyl ethers include perfluoro(2-n-propoxy-propyl vinyl ether) ##STR1## (referring to as PHVE); perfluoro(3-methoxypropyl vinyl ether) (CF.sub.2 .dbd.CFO(CF.sub.2).sub.3 OCF.sub.3); perfluoro(2-methoxypropyl vinyl ether)(CF.sub.2.dbd.CFOCF.sub.2 CF(CF.sub.3)OCF.sub.3); perfluoro(2-methoxyethyl vinyl ether) (CF.sub.2 .dbd.CFO(CF.sub.2).sub.2 OCF.sub.3) and perfluoro (3,6,9-trioxa-5,8-dimethyl dodeca-1-ene)(CF.sub.2 .dbd.CFOCF.sub.2 CF(CF.sub.3)OCF.sub.2 CF(CF.sub.3) OCF.sub.2 CF.sub.2 CF.sub.3). It is especially preferable to use PHVE in view of commercial availability and effect for a low temperature resistance. One or more of the fluorovinyl ethers can be used in the copolymerization.
In the copolymerization of the present invention, it is possible to provide the other comonomer units at a content of up to 20 mole % beside the vinylidene fluoride units and the fluorovinyl ether units.
Such other comonomers can be fluoroolefins, such as tetrafluoroethylene, chlorotrifluoroethylene, vinyl fluoride and hexafluoropropylene; olefins such as ethylene, propylene, isobutylene and butene-1; and vinyl ethers such as perfluoro(propyl vinyl ether), perfluoro(methyl vinyl ether) and ethyl vinyl ether. A copolymer having 15 mole % or less of perfluorovinyl ether units which have perfluoroalkyl group having less carbon atoms such as perfluoro(propyl vinyl ether) has advantageous mechanical characteristics.
It is preferable to incorporate comonomer units for reactive sites in the copolymer of the present invention in order to improve crosslinkability. The comonomer can be perfluoro(2-bromoethyl vinyl ether), 2-chloroethyl vinyl ether, vinyl monochloroacetate etc. In view of copolymerization reactive characteristics, and heat resistance and low temperature resistance of the copolymer and reactivity as cure sites, it is especially preferable to use perfluoro(2-bromoethyl vinyl ether) (hereinafter referring to as BVE). When BVE is used as a comonomer for reactive sites, it is preferable to have the BVE units at about 0.1 to 5 mole % especially about 0.5 to 2.5 mole % in the copolymer. When the content of the BVE units is too small, the effect as cure sites is not satisfactory, whereas when it is too much, the unreacted cure sites are remained in the vulcanized rubber whereby it increases to cause adverse effect to heat resistance and chemical resistance.
In the copolymer of the present invention, it is important to give a molar ratio of vinylidene fluoride units to fluorovinyl ether units of 95:5 to 50:50 preferably 90:10 to 60:40. When the molar ratio of vinylidene fluoride is more than the upper limit, rubber-like elasticity is lost, whereas when it is less than the lower limit, the reactivity for the copolymerization is lower to cause low molecular weight and the mechanical characteristics as the elastomer are inferior.
The copolymer of the present invention can be produced by applying a radical initiation source to a monomer mixture of vinylidene fluoride, the fluorovinyl ether, and, if necessary, the other comonomer. Such copolymerization may be carried out at a high temperature under high pressure, however, it is preferably carried out at a low temperature to obtain a copolymer having high molecular weight. The radical initiation source can be various peroxides or azo type polymerization initiators suitable for low temperature polymerizations as well as an ionized radiation.
The initiators suitable for low temperature polymerizations include organic peroxides such as diisopropylperoxydicarbonate, diethylhexylperoxydicarbonate, acetylcyclohexylsulfonylperoxide, t-butylperoxypivalate, 2,4-dichloro-benzoylperoxide, isobutylperoxide and octanoylperoxide; and azo compounds such as 2,2-azobis(4-methoxy 2,4-dimethylvaleronitrile) and a combination thereof with a desired reducing agent. It is especially preferable to use a redox initiator as a combination of an inorganic peroxide such as a persulfate and a desired reducing agent for example, a highly active redox initiator comprising a persulfate, an iron ion source, ethylenediamine-tetraacetic acid or a salt thereof and a sulfinate.
An amount of the polymerization initiator is decided depending upon a kind of the initiator and a condition for copolymerization and it is usually in a range of 0.005 to 5 weight part especially about 0.01 to 0.5 weight part per 100 weight parts of the total monomers.
The copolymer of the present invention can be produced by a batch, semi-batch or continuous polymerization process. The type of the polymerization can be a bulk polymerization, a solution polymerization or an emulsion polymerization. In view of effective production of a copolymer having high molecular weight, it is preferable to employ an emulsion polymerization.
A temperature in the polymerization is preferably in a range of 80.degree. to -30.degree. C. especially 40.degree. to 0.degree. C. When the polymerization temperature is too high, it is difficult to obtain a copolymer having high molecular weight whereas when it is too low, it causes troubles such as solidification of a medium for polymerization. A pressure in the polymerization can be as in the conventional polymerizations and it can be lower than 150 kg/cm.sup.2 especially lower than 100 kg/cm.sup.2.
The medium for copolymerization can be selected depending upon the type of the polymerization and it is preferably selected so as to possibly prevent a chain transfer.
The media for polymerization suitable for a solvent polymerization are those having a low chain transfer constant such as t-butanol, methyl acetate and chlorofluorocarbon. However, in view of an effective production of a copolymer having high molecular weight, it is advantageous to employ an emulsion polymerization using an aqueous medium. In view of the acceleration of polymerization and stabilization of latex, it is preferable to employ a mixed medium comprising water and an organic solvent. Suitable organic solvents include t-butanol, methyl acetate and trichlorotrifluoroethane. When an aqueous medium is used, it is possible to add a pH modifier and a buffering agent so as to control pH of the medium depending upon an activity of a polymerization initiator.
The copolymer of the present invention can be identified by an IR spectrum or .sup.19 F-NMR spectrum to observe IR spectrum as characteristic absorptions near 990-1000 cm.sup.-1 for &gt;CF--O-- bond; and .sup.19 F--NMR spectrum in CCl.sub.3 F base, as peaks at 92-96 ppm and 112-126 ppm for vinylidene fluoride unit (corresponding to --CH.sub.2 CF.sub.2 CH.sub.2 CF.sub.2 -- and --CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 --) and peaks at 120-130 ppm for CF.sub.2 and CF of fluorovinyl ether unit.
The copolymer of the present invention is crosslinkable by an ionized radiation or an addition of a curative such as peroxy compounds. It is effective to improve a crosslinkability by a copolymerization with a third comonomer for a reactive site and it is preferable to add a cure promoter such as polyallyl compound.
In the crosslinking process for the copolymer of the present invention, it is possible to incorporate various additives used in the conventional crosslinking process. Suitable additives include metal oxides such as magnesium oxide and lead oxide; metal hydroxides like calcium hydroxide; reinforcing fillers such as carbon balck, fine silica, clay and talc; other fillers; pigments; antioxidants and stabilizers.
When various additives are incorporated into the copolymer of the present invention, it is preferable to uniformly blend them. Such blending is attained by the conventional devices such as a rubber open mill and Banbury mixer. The operation for the blending is not critical. In usual, the additives can be dispersed well into the fluorine-containing copolymer by kneading the mixture at about 30.degree. to 80.degree. C. for about 10 to 60 minutes.
It is also possible to add the additives in a form of a suspension by dissolving and dispersing the additive in a suitable medium. It is also possible to employ a wet blending for blending them in a medium. In the latter process, a suspension can be prepared by using a mixing device such as an open roll, a ball mill and a homogenizer. It is preferable to select the optimum condition and operation in the blending process depending upon kinds of the raw materials and the additives.
The mixture can be fabricated into a form of a sheet, a pipe, a rod, a tube, an angle, a channel, a coated fabric and a coated board by continuous fabrication processes such as an extrusion, a transfer molding process, a roll-coating process, a brush-coating process and a dipping process beside a compression molding process. It is also possible to fabricate it into articles with complexed shape and special molded products such as sponge rubber by various molding and processing processes. The fabricated mixture is vulcanized by a desired vulcanizing means whereby a vulcanized elastomer is obtained from the fluorine-containing copolymer of the present invention.
An elastomer having excellent heat resistance, low temperature resistance, chemical resistance and oil resistance and also excellent alcohol resistance can be obtained from the fluorine-containing copolymer of the present invention. The elastomers can be used for various usages and objects in various fields because of such characteristics, for example, heat resistance-anticorrosive gasket, packing and O-ring; oil resistance sealing material, oil hose, tube and diaphragm; heat resistant-anticorrosive roll; steam resistant gasket; gasket for heat exchanger; and heat resistant-oil resistance electric wire especially parts for cars which contact with alcohol-containing gasoline such as fuel hose, packing and diaphragm.